1. Field of the Invention
The present invention relates generally to the field of predicting sonar performance, and more particularly to systems and methods of predicting the performance of active sonar arrays in the open ocean and in shallow water coastal sites.
2. Description of the Related Art
Sonar, a well known technology in which sound waves are used for detecting objects, has found extensive application in maritime environments. Sonar systems can be broadly divided into two types: active sonar and passive sonar. An active sonar system includes a sound source, which emits bursts of sound, and an array of receivers which listen for the reflection of the emitted sound from an object. A passive sonar system has no sound source and relies on its array of receivers to detect sound emitted by the object being sought. A passive sonar system can operate covertly but can only detect objects emitting sounds louder than the ambient noise of the environment.
Throughout the cold war, a major objective of US Naval sonar was to conduct long range, deep ocean surveillance of relatively noisy nuclear submarines, a mission suited to passive sonar systems. The end of the cold war has lead to an increased need for addressing so called Third World scenarios. These scenarios include detecting quiet diesel-electric submarines running on batteries in shallow water at much shorter ranges, typically of the order of 1 to 50 miles.
Although they do not operate covertly like passive sonar, active sonars have the advantage of providing their own energy for illuminating quiet targets. Active sonars, however, not only have to contend with the ambient noise of wind, wave action and shipping but also with non-target echoes of their own transmitted signal from the rough ocean surface, bottom boundaries and fish or other biologics present in the water column. These sources of noise related to the transmitted signal are referred to as surface, bottom and volume reverberation, and the target like contributions are commonly called clutter. In littoral waters, the scattering off the ocean floor dominates the reverberant field, while scattering off the sea-surface dominates under high sea-state conditions, and volume reverberation dominates at high-frequencies and at sites with a high fish-population density.
When operating an active sonar system to detect submerged targets in shallow, littoral waters, the sonar display may be dominated by target-like clutter caused by the complicated acoustic wave-guide conditions of the local bathometry. The sonar operator has to learn to distinguish a real target echo from the clutter caused by reverberation. A system for simulating the performance of an active sonar system therefore has to account for the reverberation.
Each component of the reverberant field has a level of stochastic clutter, i.e., back-scattering that is essentially randomly distributed, and a level of deterministic clutter, i.e., clutter that is due to specific structure within the bathymetry and is typically isotropic. Clutter from surface and volume reverberation tends to be predominantly stochastic due to the dynamic nature of the water column and atmospheric conditions. Bottom reverberation, however, tends to be dominated by the deterministic clutter because of the relatively static nature of the sea-floor. Existing methods for simulating reverberation in multi-path sonar such as, but not limited to, the system described in U.S. Pat. No. 6,002,914 issued to Weinberg on Dec. 14th, 1999 entitled “Method and Apparatus for Simulating Reverberation in a Multi-path Sonar System”, the contents of which are hereby incorporated by reference, utilize stochastic methods to simulate reverberation. They work well with surface, volume and deep water reverberation, but do not provide a good simulation of the scattering off the ocean floor that dominates the reverberation field in literal waters because they do not model the deterministic component of the scattering.